1. Field of the Invention
The present invention relates to metal oxide nanoparticles and a synthetic method thereof, and in particular to maghemite (γ-Fe2O3) particles usable as a superhigh density magnetic recording substance by having good shape anisotropy and magnetic characteristics, hematite (α-Fe2O3) nanoparticles usable as a precursor to the maghemite or as catalysts, maghemite and hematite-mixed particles and a synthetic method thereof.
2. Description of the Related Art
With the development of the telematique industry, increase of memory, miniaturization and high integration of electronic materials have been constantly required in the world semiconductor market.
According to the requirement, recording density of a commercial magnetic recording medium has been increased steadily every year. In the meantime, storing density of a magnetic recording medium is in inverse proportion to a size of a storing bit. In order to control SNR (signal to noise ratio) and other recording variables, the number of ferromagnetic particles per bit has to be great as many as possible. Accordingly, development of a finer-sized new magnetic substance having appropriate coercive force and a high magnitization value has been required. In addition, in order to maximize SNR, exchange coupling between particles has to be prevented by separating each particle, and the particle size distribution has to be in a narrow range.
However, it was discovered the recent method using nano-sized particles had a limit. In more detail, when a particle size is not greater than a specific size (˜10 nm), inherent characteristics of a substance are instantly changed into superparamagnetic, it is inappropriate for a magnetic recording medium. In particular, that phenomenon was observed in Co, Ni, Fe or spherical magnetite, maghemite and other ferrite particles, etc. having a size of 10 nm approximately [J. Am. Chem. Soc., 123, 12798(2001); J. Am. Chem. Soc., 122, 8581(2000)].
In addition, in the reported method, a synthetic method of Fe or Fe oxide or ferrite nanoparticles, etc. mostly has to be performed in a nitrogen or an argon atmosphere, and accordingly it is intricate.
In the U.S. Pat. No. 6,302,940, in order to maintain sufficient coercive force while reducing a particle size of a substance, a method for improving crystal magnetic anisotropy has been presented. In the U.S. patent, a nano-sized spherical FePt has been presented, however, in order to change the inherent paramagnetism of the spherical FePt into ferromagnetism, performing an additional heat-treatment process at a temperature not less than 550° C. is required, and oxidation easily occurs in the atmosphere. In addition, because high-priced platinum has to be used, it is not economical, and it is intricate to control a synthetic process in an inert atmosphere.
In recent researches, nano-sized cobalt particles having a rod shape have shown ferromagnetism due to shape magnetic anisotropy and are arranged in the parellel direction, and accordingly it has been researched as a future superhigh density magnetic recording substance. However, in that method, synthesized nano-particles are not stable to oxidation in the atmosphere, a high-priced organometallic compound has to be used as a precursor, a synthetic process has to be performed in an inert atmosphere, and accordingly it is difficult to control the process. In addition, nonoparticles having a spherical shape shows superparamagnetism [Science, 291, 2115(2001)].
In the meantime, maghemite (γ-Fe2O3) has been known as a substance having great magnetic anisotropy, and presently rod-shaped maghemite particles having a diameter of 0.25 μm and a length in the ratio as 6 to the diameter are used as a high density magnetic recording substance. In the synthetic process of the rod-shaped maghemite (γ-Fe2O3), first, the rod-shaped hematite (α-Fe2O3) is synthesized, and it is reduced to magnetite (Fe3O4) with the same size and shape. Next, by oxidizing magnetite carefully, the magnetite is transformed to maghemite. The maghemite is a stable oxide in the atmosphere. However, even in case of maghemite, superparamagnetism occurs in spherical nanoparticles having a size of 10 nm or less. Until now, maghemite having shape anisotropy in a size as approximately 10 nm has not been reported. In case of hematite, nanoparticles (without aggregation) not greater than 30 nm have not been reported regardless of shapes thereof. Hematite nano-particles are expected to have usability as catalysts or pigments besides a precursor to maghemite.